Disclosed are security articles and methods and systems for authenticating security articles using a plurality of stimuli. According to one embodiment, an illustrative secured article includes an embedded feature wherein differential activation of phosphorescent or fluorescent materials creates a machine readable response through the simultaneous presence of a source of electromagnetic radiation and a specific gas environment. A detector detects a differential spectral emission across the security feature that results from the change in gas environment. The spectral emission may be compared to an expected emissive signature to determine authenticity of the security article.

A method and apparatus for cleaning a stack of secure instruments is disclosed. Each secure instrument includes a substrate, visual data and a security feature. The method and apparatus include exposing the stack to a supercritical fluid at a temperature and a pressure and for a duration sufficient to clean each secure instrument and not compromise each security feature and each visual data, and maintaining a securing mechanism on the stack during exposure of the stack to the supercritical fluid such that cleaning each secure instrument includes one or more substances from each secure instrument into the supercritical fluid.

A method and system for cleaning a secure instrument, such as a banknote, including a substrate, visual data and a security feature, including exposing the secure instrument to a supercritical fluid at a temperature and a pressure and for a duration sufficient to clean the substrate and not compromise the security feature and the visual data, wherein to clean the substrate includes to remove one or more substances from the substrate into the supercritical fluid. The substances removed from the substrate may include contaminants, dirt, sebum and pathogens.

A method for enhancing the structural strength of a porous substrate having pores therein is disclosed. The method includes soaking the porous substrate in a solution having a first solvent and at least one polymer dissolved in the first solvent at a specific temperature and pressure, such that the solution is deposited within pores of the porous substrate, soaking the porous substrate in a second solvent, such that the first solvent diffuses into the second solvent, and such that the at least one polymer remains within the pores of the porous substrate, and flushing out the second solvent from the porous substrate.

Systems and methods for document and product authentication are provided using a combination of interacting absorption and emission materials that are formed into covert optically encoded markings. The markings are formed from at least one emitter and at least one absorber, that exhibit a first emission in response to a first excitation of the emitter and a change in that emission in response to excitation of the absorber such that various combinations of emitter and absorber materials create a machine readable response that cannot be detected by the human eye.